Retting method



Patented Sept. 10, 1946 BETTING METHOD Theodore Earle, Pacific Palisades, Calif.

2 Claims.

This invention relates to the conditioning and treatment of fibrous vegetable or plant material, such as flax, ramie, hemp, and the like, for the freeing and initial detachment of hast fiber from its associated material, and has as a primary object the provision of an improved method for the retting of fiber-carrying straws and analogous plant matter.

A further object of the invention is to provide an improved method applicable as an extension and refinement of conventional retting processes to expedite the latter and enhance the recovery of high quality fiber had therethrough.

A further object of the invention is to provide an improved method susceptible of convenient practice to materially shorten the time of conventional retting without adverse effect on the quantity and quality of the recovered fiber.

A further object of the invention is to provide an improved rotting method characterized by controls operable to regulate the intensity and dura- Application November 12, 1943, Serial No. 510,072

tion of the retting action, adapt the method to the treatment of either natural or decorticated fibrous plant material of various species, and determine with practical precision, the stage of treatment at which retting action should be discontinued. I

A further object of the invention is to provide an improved method operable to rapidly and efficiently ret previously. decorticated fiber.

A further object of the invention is to provide an improved retting method applicable to natural bundles of decorticated plant fibers to initially detach the individual fibers from the others and thereby facilitate subsequent separation of the individual fibers.

A further object of the invention is to provide an improved retting method that is simple, convenient and inexpensive of practice; that is susceptible of practical application through means and apparatus commonly available for conventional retting techniques; and that, in use, is productive of a relatively enhanced yield of high quality fiber.

My invention consists in the nature, character, sequence and combination of steps hereinafter set forth, pointed out in my claims, and illustrated by the accompanying drawing, in which the view is a diagram or flow-sheet illustrating the character and sequential relation of steps comprising the improved method.

The treatment of certain harvested fibrous plants, particularly flax, ramie and hemp, for detachment, separation and recovery of their bast fibers is, ina general sense, rather Well standardized after long experience as consisting of five distinct phases or stages known as rippling,

retting, grassing or drying, breaking, and scutching. These phases and stages, and their relation in point of time and in effect on the ultimate fiber, are well understood by those killed in the art of fiber production and require no elaboration herein; the improved method of the instant invention relating to and being concerned with only the retting stage or phase.

While the improved method is of advantage in the retting of various fibrous plants, it will be described and exemplified as employed in the treatment of flax, which plant material is to be considered as typical rather than limitative, and as including for purposes of this disclosure analogous material susceptible of similar reaction to the method.

Retting is that phase or stage in the treatment of fiber-carrying plant material which has as its purpose facilitation of the separation of individual fibers from the surrounding plant matter, boon, shives, pith, etcetera, and from each other, through the breaking down, or removal from the plant structure, by means of fermentation, of the gummy, resinous, or waxy binding substances present in the plant. Conventional retting methods are of two general types, ancient as to origin and but little modified in modern practice; one, dew-retting, consisting in the exposure of the plant material in the open to the uncontrolled influences of dew, rain, air, light and natural temperature variations; and the second, water-relating, consisting in steepingof the submerged plant material in natural or artificial ponds, sluggish streams, or tanks under greater or less control of water purity, chemical properties, temperature and circulation. Each type of rotting has certain advantages and disadvantages, but the two types are alike in that both are slow in efiecting the desired end, are variable in action and difiicult to control, and necessitate tactile check of the material under treatment and the exercise of trained judgment to determine, with little certainty, when the retting action has progressed to the most favorable degree and should be stopped. Since water-retting is the more extensively utilized method and is more readily subject to desirable controls, the improvements of the instant invention have been developed and will be described primarily in their 3 manner hereinafter to be set forth, with consequent shortening of the operation as conventionally practiced and with improvement in the amount and quality of fiber yield.

Ihe aim of any retting procedure is to separate the shive from the fiber, remove the lignins and hemicelluloses, and partially remove the pectins and waxes without forming any oxycelluloses, brittle hydrocelluloses, or an excessively degummed fiber. Since the chemical composition of the shive is closely similar to that of the fiber, it is obvious that the retting, for satisfactory results, should be a precision operation susceptible of close control, recognition of which fact is evidenced, from antiquity to the present day, in the selection of certain natural waters best suited to the purpose or the chemical modification of the waters available, in the preference for certain localities or geographical areas characterized at the crucial season by conditions of climate and temperature most favorable to retting or by the artificial simulation of such conditions, and by the various attempts, including the addition of cultures, to timulate and maintain fermentation. The improved method, particularly when applied to water-retting in tanks, provides the controls and standards which operate to enhance the precision, improve the yield and grade of line" fiber, and shorten the duration of the process as hitherto practiced.

Conventional retting technique is so well known and thoroughly documented as to require no review herein, and it has long been recognized that the retting action is a process of fermentation activated by certain agents carried in and natural to the plant material. It has been long considered that the agents productive of retting action were bacterial in character, and such may indeed be the case, but the more recent belief is that such agents should be classified as enzymes, whether of bacterial or other origin, inherent in the straw, or analogous portions of other plant material, and ever present for the initiation and stimulation of fermentation under proper conditions of moisture, temperature and time. Whatever be the true nature of the activating agents, which will be hereinafter referred to as enzymes, their action is clearly catalytic or enzymic as evidenced by their power to influence chemical processes without themselves being consumed or characteristically altered.

It has been long understood and is now authoritatively established that the natural plant enzymes vary in kind and in their speed and intensity of reaction under varying conditions. In the water-retting of flax, experience has shown and competent authorities state that initial activation of the retting enzymes may be hastened and their retting effect intensified by heating the water bath wherein they operate and maintaining said bath at elevated temperature throughout the retting process. The range of temperature wherein the retting enzymes function to advantage is somewhat limited, and too high a temperature will operate to arrest the action of, and perhaps destroy, such enzymes, while too low a temperature retards the enzymic action, the optimum water temperature for flax retting appearing to be 90-92 degrees F., at which point rap-id retting action was productive of unusually good yields of high quality fiber, the minimum operating temperature appearing to be at or slightly below 70 degrees F., at which point much slower retting action was productive of satisfactory results, and the maximum operating 4 temperature being in the neighborhood of 100 degrees F., above which point the retting action rapidly diminished.

Experience demonstrates that the retting enzymes thrive and intensify their activity in a definitely acid substrate and Within relatively narrow limits of acidity. Activity of the retting enzymes is productive of an increase in the acidity of the substrate up to a degree of acid concentration which clearly checks the enzymic action and retards rather than stimulates further retting, hence the need for careful and continuing control of the acidity of the retting solution throughout the retting process. Experience teaches the desirability of an initial acidity in the retting solution, where flax straw is concorned, giving a pH of 6.2 to 6.7 and a maximum ultimate acidity of about pH 4.7 to 5.0, at which latter value further steeping of the straw is productive of a decrease in the enzymic action and a reduction in the acid concentration.

Conventional retting processes depend on fermentation of the steeping straw to initially acidify the water to that degree productive of the desired enzymic concentration and action, and a considerable portion of the conventional retting time, to 144 hours, is consumed in bringing the substrate to the acid condition wherein the enzymic action is potent, and the rest of the time is utilized in intensifying the enzymic action and in applying such action to the desired retting effect on the straw, with a consequent increase in the acidity of the solution. Hence, as has to some extent been previously recognized, the use of an initially slightly acid retting solution will serve to stimulate and expedite the desired enzymic action, thereby somewhat shortening the retting time, and maintenance of the solution at the optimum temperature above noted will additionally serve to expedite the desired action and correspondingly shorten the retting time. However, even with a slightly acid substrate heated to 93-92 degrees F., considerable time elapses after the straw is submerged in the solution before the enzymes are activated to any appreciable effect, and a most important and essentially novel feature of my improved method is the use of a starter which operates to promote immediate enzymic action without the 48 hour period of gaseous fermentation characteristic of conventional methods.

Repeated tests have persuasively established the efficacy of enzymes from a previously-active retting solution as agents for activating, promoting and stimulating enzymic action on straw in a fresh solution, and such agents can best and most conveniently be introduced into the fresh solution by combining with the latter a proportioned amount of solution from a previous ret. It must be borne in mind that in the practice of the improved method the enzymic concentration of an active retting solution increases, apparently progressively, with the age of the solution, while the acid concentration and apparent enzymic action increase with the age of such solution up to a certain point, whereafter greater ag of the solution results in adecrease of acidity and enzymic action, even tho-ugh the enzymic concentration may continue to increase. Such being the case, the degree of acidity is a measure of the enzymic concentration of the solution up to the age point of the solution at which the acidity reverses, and thereafter the enzymic concentration can be gauged only as, a function of the solution age, hence proper control in the preparation of a fresh rotting solution requires a knowledge of the age as well as of the acidity of the old solution to be used. Where age of solution is referred to herein, the term relates to the length of time that the solution has been in contact with straw or other plant material to be retted, and does not include time which may have elapsed after removal of the plant material from the solution, t

My improved method of rotting combines, and utilizes in combination, the three steps of heating the solution, acidifying the solution, and adding to the fresh solution a starter from a previously-active retting solution, all in accordance with. the techniques and controls herein after set forth,

In the practice of the improved method, the exact procedure to be followed will vary according to the facilities-available. Where conventional retting tanks areto be used, it is generally desirable to charge the tanks with sheaves or beets of the straw in the usual manner, prepare the warm acid solution in a container away from the charged tank, add the correct amount of old solution to the new tank, introduce the acid solution to saidtank, and then circulate the mixed solution through and about the straw under conditions which permit control of temperature and addition, if desired, of fresh water, either acidified or not. where facilities permit, to prepare the solution in the tank to be employed and then charge the straw into the tank and solution, or even to mix and prepare the solution in the straw-charged tank wherein it is to circulate. In general, better results deriving from more precise control can be had when the natural water to be used is first thoroughly mixed with a suitable acid added in such quantity as to bring the water to .a pH of from 6.4 to 6.8 to which mixture is then added an amount of solution from a previous ret hav- 3 ing a pH of about 5.2 so that the pH of the resulting mix is from 6.2 to 6.4, and then heating the prepared solution to a temperature of about 90 degrees F. and introducing it into the strawcharged tank where it is maintained at the initial temperature noted and circulated through, over and about the strawsubmerged therein. It is best, as indicated, towarm the acidified wat r before its introduction into the tank and thereby conserve time otherwise lost in heating the complete tank contents. When the procedure above outlined is followed, active fermentation evidenced and accompanied by ebullition and the emission of gas begins in about three hours and continues for some fourteen to twenty hours additional, during which time the straw remains tough and shows little change from its original condition and the acid concentration of the solution increases. Some twenty-four hours or less after exposure of the straw to the prepared solution, ebullition subsides and, the evolution of gas ceases, but the enzymic action continues, with perhaps enhanced intensity, as evidenced by a continuing increase in the acid concentration of the solution and a marked effect on the straw, whereof the shives become increasingly brittle as the action progresses. The acid concentration of the solution will continue to rise until it reaches that maximum where, due to enzyme concentration, the phenomenon characteristic of enzymic action is made manifest by a reversal effect resulting in a decline, of the acid concentration. This reversal takes place approximately forty-eight hours after first exposure of However, it is feasible,

. covered, and further steeping of the straw tends to subject the fibrous material to enzymic action to such a degree as undesirably affects the quality of the ultimately recovered fibers.

Acidifying of the water to be used in the retting solution according to the improved meth- 001 may apparently be accomplished with any one of the more commonly available, fullstrength acids without noticeable variation in the time of ret or quality and quantity of fiber recovered. Numerous tests respectively utilizing hydrochloric, nitric, sulphuric, acetic, and other acids to initially acidify the natural water of the solution have been conducted to the success-- ful recovery of high quality fiber with no appreciable difference in result chargeable to the use of a particular acid, though for reasons of convenience, availability, and economy it has been found desirable to use a commercial grade of full-strength sulphuric acid (66 degrees Baum) as an acidifying agent throughout the typical examples hereinafter. The proportion of acid to be mixed with the natural water of the solution will, of course, vary with the initial pH of the water and the strength of the acid used, relatively very small additions of acid being sufficient to bring the resulting mixture to the desired pH when water free from excess of minerals and alkalis is used.

Determination of the amount of enzymecharged solution from a previous ret to be con bined with the acidified water for best results in a subsequent ret is not alone a matter of the pH of the resulting mixture, but should take into account, to some extent, the character of the specific straw to be treated, and to a greater extent must be governed by the enzyme concentration of the old solution. As has been previously pointed out, the acid concentration and the enzyme action and concentration will simultaneously increase in an active retting solution during the earlier stages of fermentation and until a maximum point of acid concentration has been reached, whereafter the acid concentration and enzyme action decrease while the enzyme concentration increases as fermentation and retting is permitted to continue- Since addition of old solution to the acidified water of a new solution has for its purpose the provision of a controlled initial enzyme concentration in the new solution, the pH of the old solution must be read with reference to the active age of the old solution in determining the enzyme concentration indicated by such pH. When the old solution is approach ing, or has reached but not yet passed, its point of maximum acid concentration, its decrease in pH value is a fair measure of its enzyme concentration, but when the old solution has passed its point of maximum acid concentration, then its increase in pH value is a measure of its increase in enzyme concentration beyond that ohtaining at the point or time of its maximum pH.

Thus, two identical pH values of the same old solution, one attained before and the other after the point of maximum acid concentration there in, will representtwo entirely different degrees of enzyme concentration.

It is clear from the foregoing that less of an old solution that has passed its point of maximum acid concentration will be required to provide a given enzyme concentration in the new solution than will be the case when the old solution has not yet attained its point of acid concentration reversal, so that, in practice, the volume of old solution added to the acidified water of a new solution may vary from five to thirtyfive percent of the volume of fresh water. Too high an enzyme concentration in the new solution appears to retard the ret and hould be avoided, the best condition seemingly being one where the old solution provides sufiicient enzymes to initiate immediate retting action and thereby more quickly stimulate the development of more enzymes from the plant material, but does not provide enough to accomplish the retting and, by saturating the solution, retard the enzymic action and development deriving from the material being treated. Also, use of old retting solution that has stood for some time, ninetysix hours or more, is preferably to be avoided, since such old solution becomes very weak in acid, is congested with enzymes, may be sour, and tends to produce discolored, harsh fiber of inferior quality. The old solution should preferably be taken from a just-completed ret and be at or very close to its point of minimum pH, or maximum acid concentration, in which case the effect of the added old solution on the pH of the new solution may be used in determining the amount of such old solution to be employed.

Flax straws and analogous plant materials vary considerably in their reactions to given retting solutions, due, perhaps, to differences in soil and climatic conditions obtaining during their growths, but simple experiments performed on small amounts of the materials to be treated will serve to readily determine the variations in solution proportions and techniques for best results in the retting treatment.

While the greatest economy of time is had when, in the practice of the improved method, the temperature of the solution is initially raised to 90 to 95 degrees F. and maintained at approximately 90 degrees F. throughout the retting operation, it is notable that the other steps of the improved method are productive of a relatively great saving of time and an enhanced recovery of high quality fiber when practiced without heating of the solution above the normal temperature of the water used. Starting with a natural water having a temperature of 70 degrees F., acidifying of the water and the addition of old retting solution, in accordance with the foregoing teaching, results in completion of a satisfactory ret in from five to eight days, whereas conventional practices operate at the same solution temperature to complete satisfactory retting only after fourteen days, at best, and frequently require even longer periods of time on the order of thirty days or more. In general, the use of the unheated solution is disadvantageous only in the greater length of time required for retting, and the results obtained, so far as yield and quality of fiber are concerned, compare very closely with those had through the use of the heated solution in a like method, but there is a tendency of the solution to become sour after ninety hours or so, with adverse effect on the yield and quality of fiber, unless great care is exercised to maintain proper circulation and aeration of the solution and to add proper amounts of fresh solution at suitable intervals. In acidifying the unheated solution greater amounts of acid may be employed to advantage without impairing the speed of the ret or the quality or yield of fiber. Due perhaps to the slower rate of enzyme development and activity in the unheated solution, a higher initial acid concentration in the solution, resulting from the addition of up to three times the amount of acid employed in a heated solution of like volume, is practically helpful in stimulating enzymic action and in shortening the retting time.

The novel technique of the improved method are advantageously adaptable to the detachment and individual separation of bast fibers in natural bundles from which the shive has been removed by mechanical or other decorticating means distinct from retting. While it has been found practical to almost entirely remove the shive from flax and analogous straws without retting or steeping, no procedure other than retting has as yet proved adequate for releasing the bond that hold the individual fibers together in the decorticated bundles, and the removal of the shive material complicates retting of the decorticated bundles by conventional methods. Tests have persuasively established the fact that the greater part of the retting enzymes are intimately associated with and derive from the shive material of the straw to such extent as to delay the development of any retting action when straw divested of its shive is subjected to conventional retting practices. Decorticated fiber bundles substantially free of shive material were steeped in acidified water for a period of two days with little indication of any fermentation whatever and with no change in the bond uniting the fibers, but when a small amount of enzyme-charged solution from a previous straw ret was added there was strong fermentation after a stand of four hours and a yield of commercial retted fiber after thirty hours.

The procedure above outlined for the retting of whole straw needs little modification for successful retting of decorticated fiber. Prior to retting of the decorticated material it is of advantage, and some times practically necessary, to wash the fiber bundles for removal of coloring matter and for conditioning of the material to be retted. The washing may be done in either normal temperature or warm water and may involve soaking of the material for a considerable period or repetitious washings in a series of waters, a the character and condition of the material may indicate. The wash water may be slightly acidified, particularly if the natural water is abnormally alkaline, and it is desirable to largely free the washed material from its wash water, by s ueezing or centrifuging, before it is placed in the retting solution. Preparation of the solution for the retting of the washed, decorticated fiber material differs but little from preparation of the solution for straw retting. However, the water to be used can be more highly acidified, even to a pH of 4.0, then preferably heated to a temperature of to degrees F., and admixed with a suitable amount of correspondingly heated solution from a previous ret. The old retting solution used is preferably, but not necessarily, taken from a straw ret rather than a decorticated fiber ret to insure proper concentration of active, potent enzymes, and should be relatively fresh, and the amount of old solution to be employed in a given instance will vary up to fifty percent, by volume of the com- 9 pleted solution, depending on the amount of shive left on the decorticated material and the potency of the enzyme concentration in the old solution, it being obviou that any shive carried by the material to be retted is itself ,a source of enzymes requiring only the presenceof a starter to supply much of, the enzymic action and fermentation needed. Where solution from a recent straw ret is not available, the required starter solution may be developed through the retting of detached shives, or a'bed of shives may be placed in the retting tank beneath the fiber bundles and disposed for the circulation of the acidified water therethrough. In the latter case, of course, the retting time will be increased by that amount necessary to initiate enzyme development in and from the shives, there being no starting charge of active enzymes present, hence previous prep aration of a starter solution is to be desired.

Given a solution properly prepared in accordance with the principles above set forth, retting of the decorticated material may normally be completed with a yield of high quality fiber greater than is had through other known methods in a total time of not more than twenty-four to thirty-six hours.

Upon conclusion of the desired retting action, performed on either whole straw or decorticated fiber bundles, the ret solution is drawn off from the treated material and the latter may be washed with fresh water and then allowed to thoroughly drain, whereafter, in accordance with usual practice, the drained material is stacked and spread in the open until dry. During the drying period it is probable that some further retting occurs, especially interiorly of the sheaves or bundles, until the lack of moisture and the effect of ultra-violet rays entirely nullify enzymic action. Alternative to the above procedure, the

drained plant material may be centrifuged for the removal of solution and dried in artificiallygenerated currents of air, or stacked in the open for relatively slower drying, with material saving in the time required for conditioning of the fiber for subsequent breaking and scutching.

The complete method hereinabove detailed, as applied to both natural straw and decortioated fiber bundles, is. graphically exemplified by the flow-sheet or diagram of the drawing, wherein the circuit of the material subjected to the method is traced progressively through the successive steps or phases constituting the method in a manner clearly portraying the nature and character of, and the agents cooperatively comprising, said steps or phases, direction of material progress and introduction into the flow cycle of method-essential agents and techniques being conventionally represented by arrows.

Typical of the techniques employed and the results obtained in treating various straws and decorticated fibers in accordance with the principles of the improved method, the following test data emphasizes the simplicity and advantage of the method.

Except when and as otherwise specified, all of the tests were made on like samples of flax straw grown in Oregon, using tap water, sulphuric acid of 66 degrees Baum, old solution from a prcvious straw ret, and a solution temperature of 90 to 92 degrees. At conclusion of each test the solution was drawn off, the retted material centrifuged, dried in the open, and sent to a commercial mill for breaking and scutching.

Test No. 1. Control test.--Conventional procedure without addition of acid or old ret solution.

Time of ret, g Developments noted hours 0 7. 5 (Tap water.) 24 6. 0 Strong ebullition of gas. 48 5.6 Ebullition of gas over at 38 hours. 72 5. 3v Straw begins to get brittle. 96 6. 3 Shives separate well. Weak fiber.

Test No. 2. Control test.Addition of acid only Time of ret, g Developments noted hours 0 8. 1 (Tap water.) 0 6.8 Addition of acid. 24 6. 1 Continuing ebullition of gas. 48 5. 4 Gas ebullition ceases. 52 5. 2 Straw removed for comparative purposes.

Milled fiber from Test No. 2 was 21 percent of original dry straw weight, was rather harsh, not

considered line fiber, and needed further retting time.

Test N0. 3. Improved method-Addition of acid and old straw ret solution.

Time of ret, g Developments noted hours 0 7.6 (Tap water.)

0 6. 5 Addition of acid. 0 6.3 Addition of 7.8 per cent by volume of old sol.

having pH of 5.5.

24 5. 9 Gas ebullition over in 22 hours.

52 5. 3 Straw removed-reversal of acid concentration.

Milled fiber from Test No. 3 was 25.8 per cent of original dry straw weight and of commercial line quality, but qualitycould have been improved slightly by about two hours more of ret- Milled fiber from Test No. 4 wa 26 percent of original dry straw weight and of high grade, commercial line quality.

A test similar to Test Nos. 3 and 4 was run on a sample of Georgia-grown flax straw with results closely the same as were obtained on the Oregon-grown straw except that the reversal of acid concentration took place at about 60 hours in the case of the Georgia-grown straw, indicating the need of correspondingly longer time for satisfactory retting thereof.

The principles of the improved method can be applied to advantage when dew retting is resorted to instead of tank retting, since the straw as spread or stacked in the open can be initially sprinkled or moistened with an acidified starter solution prepared as above set forth and, if necessary, subsequently sprinkled with the same or a weaker solution. By such treatment initiation 1 1 of retting action can be materially expedited and the total time of ret cut at least in half.

Since many changes, variations, and modifications in the degree, intensity, and duration of the factors and relationships constituting the new method may be had without departing from the spirit of the invention, and may in fact be necessary in adapting the new method to the successful treatment of certain plant materials, I wish to be understood as being limited solely by the scope of the appended claims, rather than by any details of the foregoing disclosure.

I claim as my invention:

1. The method of retting flax and analogous fibrous plant material which consists of positively acidifying water to a degree approaching and slightly less than the acid concentration optimum for enzymic activity, adding thereto such amount of enzymically-active solution from a previous ret as will bring the acid concentration of the resulting mixture to said optimum value, heating said mixture to a temperature conducive to maximum enzymic action, steeping the material to be retted in said mixture at sustained elevated temperature, and removing the material from said mixture for termination of retting action when the acid concentration of the solution begins to decline.

2. The method of retting flax and analogous fibrous plant material which consists of positively acidifying water to a pH of approximately 6.6, adding thereto such amount of enzymicallyactive solution from a previous ret as will bring the pH of the resulting mixture to a value Of approximately 6.3, heating said mixture to a temperature of approximately 91 degrees F., steeping the material to be retted in said mixture at sustained elevated temperature, noting the variation of mixture pH value during the retting operation, and removing the material from said mixture for termination of retting action when the acid concentration of the solution begins to decline.

THEODORE EARLE. 

